524 Prof. Tyndall on the Action of Free Molecules on 



made at Madras between the 4th and the 25th of March, 1850, 

 are particularly suitable for the illustration of this law of 

 action. During the period referred to " the sky remained 

 remarkably clear, while great variations of the quantity of 

 vapour took place." Here are the results as tabulated by 

 General Strachey: — 



Tension of vapour. 



•888. 



•849. 



•805. 



•749. 



•708. 



Fall of temperature from "1 fi0 . n 

 6 h 40 m p.m. to 5* 40 a.m. / D u 



7°1 



8°-3 



8°-5 



l0°-3 



Tension of vapour. 



•659. 



•605. '654. 



•435. 



Fall of temperature from "1 

 6 h 40 m p.m. to 5 h 4(T a.m. J 



12°-6 



12°-1 J 13°1 



16-5. 



These results, if correct (and I am not aware that they have 

 ever been questioned), show in the most impressive manner 

 the influence of the aqueous vapour of our atmosphere on our 

 planet's radiation. As the vapour diminishes, the door opens, 

 which permits the escape of the earth's heat. The halving of 

 the vapour-tension nearly trebles the refrigeration of the ther- 

 mometer. 



Equally clear is the evidence given by General Strachey as 

 to the action of aqueous vapour upon the radiation of the sun. 

 Here are the results: — 



Tension of vapour. "824. 



•737. 



•670. 



•576. -511. 



•394. 



Rise of temperature from 1 | , -0.4. 

 5* 40" a.m. to l h 40 m p.m. J 



15°-1 



19°-3 



22P. 2 24 c 3 



27"0 



This table is the exact complement of the last. There the 

 fall of temperature was powerfully promoted by the with- 

 drawal of the vapour. Here the rise of temperature is power- 

 fully promoted by the same cause*. 



* Mr. Hill, Meteorological Reporter for the North-west Provinces of 

 India, in a paper recently presented to the Royal Society, describes an 

 attempt to determine the " Constituent of the Atmosphere which absorbs 

 Radiant Heat." He uses for this purpose the careful observations made 

 by Messrs. J. B. X. Hennessey and TV. H. Cole, at Muasoree and Dehra 

 respectively. From the absence of symmetry in the quantities of heat 

 received by the actinometer on both sides of noon at Mussoree, and from 

 the existence of this symmetry at Dehra, he infers the periodic lifting and 

 lowering of the absorbing constituent above and below the higher station. 

 He finds the variation of the absorption-coefficient to follow the variations 



